The present invention relates to a gas laser comprising a pair of elongated electrodes arranged to define a discharge region between two opposing surfaces of said elongated electrodes, wherein the discharge region defines a longitudinal axis, a wide axis, and a narrow axis. The gas laser further comprises a lasing gas disposed in said discharge region, an excitation means for energizing the electrodes to excite the lasing gas, a first mirror arranged in front of a first end of the pair of elongated electrodes, wherein the first mirror has a first distance from the first end along the longitudinal axis. Furthermore, the gas laser comprises a second mirror arranged in front of a second end of the pair of elongated electrodes.
Gas lasers like the one indicated above are generally known in the art and are commonly referred to as gas slab lasers. Recently, this type of gas laser has steadily gained in importance due to fact that this laser device is compact yet capable of producing high output powers. This important feature resides in the fact that the discharge region containing the laser active gas is defined by a pair of opposing electrodes, commonly provided as rectangular planar plates, which, in turn, define a longitudinal axis substantially coinciding with the light propagation direction, a wide axis perpendicular to the longitudinal axis with a dimension of the discharge region in the wide axis of a few centimeters, and a narrow axis defined by the two opposing surfaces of the electrodes and typically having a dimension of a few millimeters. Thus, the transverse dimension in the wide axis is significantly larger than the dimension in the narrow axis. Contrary to laser devices having a rotational symmetrical arrangement along the longitudinal axis, the gas laser of the slab type offers a relatively large surface with respect to its discharge volume defined by the electrode plates so that heat dissipation is strongly improved, thus allowing a relatively high output power with a compact construction.
Many applications require a finely focusable laser beam and, thus, an output beam of high spectral mode purity is necessary. Therefore, a gas slab laser device operating in the fundamental transverse radiation mode, i.e., the radiation mode with a substantial Gaussian beam profile in the wide and narrow axes, is highly desirable. Accordingly, many gas lasers of the slab type are configured such that the distance of the electrodes defining the height of discharge region is selected to approximately 2 mm, at most, to establish a radiation exhibiting the fundamental waveguide mode in the narrow axis, whereas “external” resonator mirrors provide for the fundamental radiation mode in the wide axis, as is the case in a gas laser having a cylindrical configuration. Due to the confinement of the radiation field by the electrode surfaces light propagation along the narrow axis is commonly referred to as “waveguide” propagation, while light propagation along the wide axis, although controlled by the external resonator mirrors, is often referred to as “free space” propagation. Preferably, a negative branch unstable resonator formed by the external resonator mirrors in the wide axis is used, wherein one of the resonator mirrors is shorter than the other resonator mirror so that the laser beam broadened according to the magnification factor of the resonator can be coupled out of the resonator. To obtain the waveguide propagation with the fundamental transverse radiation mode with respect to the narrow axis, plane parallel electrode surfaces having an excellent reflectivity with a distance of approximately not more than 2 mm are required to achieve the desired output beam of high spectral mode purity. Due to the restriction with respect to the gap distance along the narrow axis to about 2 mm to generate a fundamental mode radiation, the volume of the laser active region, and thus the available output power, is also limited.
For example, U.S. Pat. No. 5,123,028 discloses a slab laser having planar highly reflective opposing electrode surfaces that are spaced apart 2 mm. The resonator is designed to form an unstable negative branch scheme, wherein the spherical resonator mirrors are positioned at a distance of approximately 20 mm away from the electrodes. The distance of the resonator mirrors is selected such that the curvature of the wave front of the radiation at the resonator mirrors matches the curvature of the resonator mirrors to thereby reduce radiation losses.
Theoretical as well as experimental results published in, for example, “Area Scaling Boots CO2 Laser Performance”, by D. R. Hall and A. J. Baker, in Laser Focus World, Vol. 25, No. 10, pages 77-80, 1989, “High Power CO2 Waveguide Laser of the 1 kW Category”, by R. Novak, H. Oppower, U. Schaefer at al., Proc. SPIE, 1276, CO2 Lasers and Applications, II, pages 18-26, 1990, and “Planar Waveguide, 1 kW Cw Carbon Dioxide Laser Excited by a Single Transverse RF-Discharge”, by H. D. Collay, H. J. Baker, D. R. Hall, Applied Physics Letters, Vol. 61, No. 2, pages 136-139, 1992, indicate that the main advantages of CO2 lasers of the slab type are realized for a height of the discharge region of about 2 mm when the laser gas excitation frequency lies within the range of 81-125 MHz. It has been shown in these documents that the optimal design of a gas laser of the slab type requires the parameters of frequency and height to be selected in conformity with each other, whereby the specific values selected strongly affect the size of the laser head and the reliability and manufacturing costs of the laser device.
On the other hand, in “700 Watt Diffusion cooled, Large Area, 40.68 MHz Excited CO2 Laser Employing Split Wave Hybrid Confocal Resonator”, by P. Vitruk, J. Schemmer, S. Byron, Proc. SPIE, 3343, pages 677-686, 1989, it is shown that it is reasonable to excite a high power CO2 laser of the slab type in the KW power range with an RF frequency of 40.68 MHz. Additionally to the general permission for this frequency for technological applications, the use of this RF frequency remarkably simplifies the RF generator and reduces the accuracy requirements regarding the laser head design. With a smaller RF frequency, and thus a larger RF wavelength, it is more easily practicable to provide a uniform excitation along the longitudinal length of the discharge region than for the above-identified frequency range of 81-125 MHz. This property additionally gains in importance as the length of the discharge region increases, as is the case in high power slab lasers having a length of the discharge region in the order of 1 m. In the above-identified documents, it is also shown that for a frequency of 40.68 MHz, the height of the discharge region should be selected with respect to output power optimization to about 4 mm or more. As previously explained, however, the arrangement of the discharge region having two plane-parallel electrodes defining a height of 4 mm and more does not allow to stabilize the fundamental transverse waveguide radiation mode without additionally excite the third radiation mode with an intensity comparable to the fundamental radiation mode.
However, as is shown in “The Characteristics and Stability of High Power Transverse Radio Frequency Discharges for Waveguide CO2 Slab Laser Excitation”, by P. Vitruck, D. R. Hall, H. J. Baker, Applied Physics Letters, Vol. 25, pages 1767-1776, 1992, the employment of radio frequencies lower than approximately 35 MHz is disadvantageous in that the discharge transformation becomes increasingly unstable since, for high power applications, the large specific energy deposition leads to large discharge currents and a high average electron energy.
A further problem involved in conventional high power gas slab lasers of the slab type resides in the fact that for a high generation efficiency, the resonator mirrors are required to be closely positioned at the longitudinal ends of the electrodes to reduce the resonator losses owing to the coupling losses of the radiation propagating from the free space between the electrode and the resonator mirror back into the discharge region. Accordingly, a typical distance of the resonator mirrors from the electrodes is in the range from about 5-15 mm. The closely arranged resonator mirrors, however, are then exposed to plasma ions, and the interaction of the plasma ions with the mirror surfaces significantly reduces the lifetime of the mirrors while also leading to a gradual reduction of the laser output power.